#include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; #define N (1000000000+7) //#define N 998244353 #define INF 1e16 typedef long long ll; typedef pair P; typedef pair Q; const int inf = (int)1e9; ll gcd(ll a, ll b) { if (b > a) { ll tmp = b; b = a; a = tmp; } if (a%b == 0)return b; else return gcd(b, a%b); } int main(void){ ll n; cin>>n; printf("%.10lf\n",(double)7*n/2); return 0; }